Known in the art is a device for the remote control of current collectors (SU, A, 150933), comprising a two-wire communication line leading to a control desk, a bridge circuit for measuring impedance in the communication line which circuit has a current amplifier in the diagonal thereof which is connected to a first winding of a magnetic member having its second winding connected to a power supply circuit of the bridge measuring circuit and to one wire of the communication line being tested.
However this device has not come into use because the magnetic member is difficult to manufacture under batch production conditions, the accuracy in measuring impedance in the communication line is influenced by the ambient temperature, and operation is unstable when pulse noise caused by transient conditions when starting electrical motors appears in the communication line.
Known in the art is a device for remote control of an electrical apparatus (SU, A, 928529), comprising a control desk, a communication line, a power supply connected to the control desk and communication line, two current amplifiers connected to the power supply and built around two transformers. The device also comprises two units for testing the communication line for condition, each unit being in the form of an electromagnetic relay having its inputs connected to the power supply and its outputs connected to the current amplifiers. Connected to the power supply is a current comparison unit in the form of an electromagnetic relay having two windings having a normally closed output contact, two coupling diodes, three resistors and two capacitors which are connected to the power supply via the coupling diodes. Starts leads of the two winding relay coils are connected to one wire of the communication line via oppositely connected diodes and to the other wire of the communication line via the capacitors.
A logical circuit of the device is built around an electromagnetic relay and connected to the power supply, an actuating unit being connected to an electrical apparatus being controlled.
Upon a fault of the actuating unit elements resulting, e.g., from the relay failure, adhesion of contacts or failure of return springs, there is a danger of inadvertent switching on of the electrical apparatus being controlled, or it may become impossible to switch it off. Furthermore, an inadvertent actuation of the current comparison unit and actuating unit may occur when voltage is supplied to the circuit or after opening of the contacts which shunt the communication line and which are practically used as control contacts. This can be explained by the fact that in the case when voltage is supplied to the communication line during a half-cycle when a terminal diode is blocked, one capacitor of the current comparison unit is substantially instanteneously charged to the voltage of the communication line while voltage across the other capacitor of the unit is equal to zero.
The two winding relay which tests the capacitors of the current comparison unit for serviceability also makes the circuit more complicated and impairs reliability of its operation. A change in the capacity of these capacitors during operation may cause false operation of this relay under transient conditions because of the different rate of change in current in the circuits of the relay windings. Furthermore, there are considerable variations in current for switching on/off the electromagnetic relay which substantially complicates adjustment of the current comparison unit.